This invention relates to a sheet or pre-coated blank comprising a steel substrate for heat treatment, overlaid on at least a part of at least one of its principal faces, by a pre-coating having, at least one layer of aluminum or aluminum alloy overlaid, on at least a part of the above-mentioned pre-coating, by a polymerized layer having a thickness between 2 and 30 μm composed of a polymer that does not contain silicon, and the nitrogen content of which, is greater than 1% by weight expressed in relation to the above-mentioned layer, wherein the above-mentioned polymerized layer contains carbon pigments in a quantity between 3 and 30% by weight, expressed in relation to said layer.